This application relates generally to a burner assembly and more particularly to an improved burner assembly which operates in a manner to stabilize the air flow to the burner and reduce the formation of nitrogen oxides as a result of fuel combustion.
Considerable attention and efforts have recently been directed to the reduction of nitrogen oxides resulting from the combustion of fuel, and especially in connection with the use of coal in the furnace sections of relatively large installations such as vapor generators and the like. In a typical arrangement for burning coal in a vapor generator, several burners are disposed in communication with the interior of the furnace and each operates to burn a mixture of air and pulverized coal. The burners used in these arrangements are generally of the type in which a fuel air mixture is continuously injected through a nozzle so as to form a single relatively large flame, and combustion-supporting, or "secondary" air is introduced towards the burner outlet to insure complete combustion. As a result, the surface area of the flame is relatively small in comparison to its volume, and therefore the average flame temperature is relatively high. However, in the burning of coal, nitrogen oxides are formed by the fixation of atmospheric nitrogen available in the combustion supporting air, which is a function of the flame temperature. When the flame temperature exceeds 2800.degree. F., the amount of fixed nitrogen removed from the combustion supporting air rises exponentially with increases in the temperature. This condition leads to the production of high levels of nitrogen oxides in the final combustion products, which causes severe air pollution problems.
Nitrogen oxides are also formed from the fuel bound nitrogen available in the fuel itself, which is not a direct function of the flame temperature, but is related to the quantity of available oxygen during the combustion process.
In view of the foregoing, attempts have been made to suppress the burner and flame temperatures and reduce the quantity of available oxygen during the combustion process and thus reduce the formation of nitrogen oxides. Attempted solutions have included techniques involving two-stage combustion, flue gas recirculation, the introduction of an oxygen-deficient fuel-air mixture to the burner and the breaking up of a single large flame into a plurality of smaller flames. However, although these attempts singularly may produce some beneficial results they have not resulted in a reduction of nitrogen oxides to minimum levels. Also, these attempts have often resulted in added expense in terms of increase construction costs and have lead to other related problems such as the production of soot and the like.
Other attempts to improve the overall combustion efficiency of the burners include the provision of a vane, or the like, on the outer surface of each burner housing to stabilize the flow of the secondary air towards the burner outlet and into the furnace opening in which the burner is installed. However, these vanes often cause downstream eddy currents to form which suck some of the coal discharging from the burner outlet back towards the vane and compromise the performance of the burner.